Table 1. Stocking biomass recommended by Van der Lingen (1959a, b) for tilapia cultured
in ponds with different inputs and the calculated fish densities (assuming an average
weight of 38 g).Stocking Calculated
biomass density
Inputs (k~lha) Ifishlha)None (natural feeding, no fertilization) 56-112 1,475-2,950
Fertilized ponds 112-224 2,950-5,900
Fertilized ponds with supplementary feeding 560-1,680 14,750-44,200were calculated, taking into account an average weight of 38 g. If all the
fingerlings stocked accordihg to these recommendations survive by the time
carrying capacity is attained and no "wild" spawning occurs, the average
individual weight at harvest (carrying capacity/density) will be 139 to 417 g,
which is a good market weight. However, if fry of 1 to 2 g are stocked, as is
common in many countries, the average weight at harvest under the same
conditions will be only 7 to 22 g. In Van der Lingen's (195913) experiments
only 15 to 30% of the harvested fish reached a market size of over 224 g.
The densities given in Table 1 seem therefore to be too high since they will
not permit rapid growth. In Israeli ponds, where protein-rich pellets are fed,
tilapia are stocked at densities of 3,000-5,00O/ha.
Allison et al. (1979) experimentally cultured unsexed S. aureus in concrete
tanks (surface area 0.002 ha) at very high densities: 50,00dj 100,000,
200,00O/ha. "Wild" spawning decreased with increasing density (from
222,9001ha in the lowest to 38,3801ha in the highest density) and yield was
high (from 1 t/ha in the lowest to 17.3 t/ha in the highest density). Allison
et al. (1979) do not give the rate of growth but calculating from the yield
and the density, it seems to be very low. The extrapolation of such data for
application in commercial fish ponds is very questionable.
Yashouv (1969) has demonstrated that in a polyculture of S. aureus,
common carp (Cyprinus carpio) and grey mullet (Mugil cephalus), the
tilapia (if stocked below 5,00O/ha) do not affect the growth of the carp
but in many cases even stimulate it. The growth of the tilapia is also not
affected by the presence of the common carp or mullet at densities up
to 2,500 to 3,000lha. These synergistic effects my be explained by the
increased amounts of detritus through the presence of the common carp and
its consumption by the tilapia. The detritus carries dense populations of
bacteria and protozoa (Odum 1968) and constitutes a nutritious food
for the tilapia. On the other hand, the consumption of the detritus by the
tilapia improves the oxygen regime for the benefit of the common carp. The
increased yield in the polyculture pond explains why most, if not all, young-
of-the-year tilapia culture in Israel is done in polyculture systems.
Polycdture may have an additional advantage. Since common carp and
grass carp (Ctenopharyngodon idella), when large enough, can prey to some
extent on tilapia fry spawned in the pond (Spataru and Hepher 1977), they
can, therefore, help to alleviate the problem of "wild" spawning that may